1. Field of the Invention
The present invention relates to a thin film forming apparatus, and more particularly to an improvement of a sputtering apparatus for forming thin films.
2. Description of Related Art
Generally, a thin film forming apparatus has been used as means for formation of thin films of various materials in a wide range of technical fields inclusive of semiconductor technologies. Especially, a magnetron sputtering apparatus or a facing-target sputtering apparatus has been widely used in order to realize high-speed and low-temperature sputtering or to solve problems such as bombardment of a substrate with negative ions, .gamma. electrons, etc., emitted from a target.
In the magnetron sputtering apparatus or the facing-target sputtering apparatus, it is required that a magnetic field of one hundred to several hundred gauss is formed by a magnetic field generating device provided in rear of an electrode in order to trap .gamma. electrons. However, this magnetic field exerts an influence on not only the vicinity of a target but also the surface of a substrate on which a thin film is to be formed, especially on the direction of formation of magnetic domains when a magnetic film is formed. Also, in the case where the size of electrodes is made large or a distance between electrodes is widened in order to enhance the throughput, a larger magnetic field generating device is required, thereby expanding an area on which the magnetic field exerts an influence.
On the other hand, the diode sputtering apparatus has been used to form a magnetic film or a protection film for the magnetic film. This apparatus can be implemented with a simple overall construction.
The above-mentioned various sputtering apparatuses are discussed by, for example, "THIN FILM FORMATION TECHNIQUES USING SPUTTERING METHOD" published by Keiei Kaihatsu Center on June 7, 1985. The facing-target sputtering apparatus is disclosed by JP-A-62-196369, etc.
However, the conventional diode sputtering apparatus is inferior to the other sputtering apparatuses in respect of the film formation rate. Therefore, it is necessary to improve the throughput of a film forming step. For that purpose, there may be considered a method in which the throughput is improved by making the size of electrodes large to increase the number of substrates which can be placed thereon. In such a case, however, the diffusion of a plasma into the outside of two electrodes having opposite polarities becomes remarkable, as shown in FIG. 3. Namely, the plasma density N.sub.e indicated by the ordinate is rapidly lowered with the increase of a distance from the center portion, as shown in FIG. 3, so that a thin film formed is thick at the center portion but becomes rapidly thin at the outside. This tendency is remarkable as an electric power supplied is increased.